This invention is in the field of crosslinked polymers, in particular methods for stress-relief in crosslinked polymers and materials resulting from the methods.
Cross-linked, gelled polymers have an “infinite” molecular weight and are described as thermosets, implying a network that cannot be melted or molded (Billlmeyer, F. W., 1984, Textbook of Polymer Science, 3rd ed., Wiley, New York 436). This description is true for most chemically cross-linked polymers; however several cross-linked networks are known to undergo bond cleavage or depolymerization at high temperatures or under various chemical or other treatments (Adhikari, B. et al., Prog. Polym. Sci., 2000, 25, 909). Although such treatments are useful for recycling purposes, there is an associated degradation in the mechanical properties of the polymers.
“Crack-healing” networks, such as those that use groups in the polymer backbone able to undergo thermoreversible Diels-Alder reactions, are able to relieve stress without mechanical degradation (Chen, X. et al., 2002, Science, 295, 1698). These reactions must be performed at elevated temperatures, making them unsuitable for thermally sensitive applications.
Reversible bond cleavage is also known to occur during some polymerization processes. Some controlled free radical polymerization techniques employ degenerative chain transfer agents such as reversible addition-fragmentation chain transfer (RAFT) agents and iniferters. As part of the RAFT process, the RAFT agent adds to an active chain. At that point, the RAFT agent may fragment, with part of the RAFT agent remaining attached to the end of the chain (now dormant) and the other part forming a new radical. Similarly, in processes employing iniferters, iniferter fragments attached to chains are attached at the chain ends.